The art of drilling vertical, inclined, and horizontal holes plays an important role in many industries such as the petroleum, mining, and communications industries. In the petroleum industry, for example, a typical oil well comprises a vertical borehole which is drilled by a rotary drill bit attached to the end of a drill string. The drill string is typically constructed of a series of connected links of drill pipe which extend between surface equipment and the drill bit. A drilling fluid, such as drilling mud, is pumped from the surface through the interior surface or flow channel of the drill string to the drill bit. The drilling fluid is used to cool and lubricate the drill bit, and remove debris and rock chips from the borehole created by the drilling process. The drilling fluid returns to the surface, carrying the cuttings and debris, through the space between the outer surface of the drill pipe and the inner surface of the borehole.
Conventional drilling often requires drilling numerous boreholes to recover oil, gas, and mineral deposits. For example, drilling for oil usually includes drilling a vertical borehole until the petroleum reservoir is reached. Oil is then pumped from the reservoir to the surface. As known in the industry, often a large number of vertical boreholes must be drilled within a small area to recover the oil within the reservoir. This requires a large investment of resources, equipment, and is very expensive. Additionally, the oil within the reservoir may be difficult to recover for several reasons. For instance, the size and shape of the oil formation, the depth at which the oil is located, and the location of the reservoir may make exploitation of the reservoir very difficult. Further, drilling for oil located under bodies of water, such as the North Sea, often presents greater difficulties.
In order to recover oil from these difficult to exploit reservoirs, it may be desirable to drill a borehole that is not vertically orientated. For example, the borehole may be initially drilled vertically downwardly to a predetermined depth and then drilled at an inclination to vertical to the desired target location. In other situations, it may be desirable to drill an inclined or horizontal borehole beginning at a selected depth. This allows the oil located in difficult-to-reach locations to be recovered. These boreholes with a horizontal component may also be used in a variety of circumstances such as coal exploration, the construction of pipelines, and the construction of communications lines.
While several methods of drilling are known in the art, two frequently used methods to drill vertical, inclined, and horizontal boreholes are generally known as rotary drilling and coiled tubing drilling. These types of drilling are frequently used in conjunction with drilling for oil. In rotary drilling, a drill string, consisting of a series of connected segments of drill pipe, is lowered from the surface using surface equipment such as a derrick and draw works. Attached to the lower end of the drill string is a bottom hole assembly. The bottom hole assembly typically includes a drill bit and may include other equipment known in the art such as drill collars, stabilizers, and heavy-weight pipe. The other end of the drill string is connected to a rotary table or top drive system located at the surface. The top drive system rotates the drill string, the bottom hole assembly, and the drill bit, allowing the rotating drill bit to penetrate into the formation. In a vertically drilled hole, the drill bit is forced into the formation by the weight of the drill string and the bottom hole assembly. The weight on the drill bit can be varied by controlling the amount of support provided by the derrick to the drill string. This allows, for example, drilling into different types of formations and controlling the rate at which the borehole is drilled.
The direction of the rotary drilled borehole can be gradually altered by using known equipment such as a downhole motor with an adjustable bent housing to create inclined and horizontal boreholes. Downhole motors with bent housings allow the surface operator to change drill bit orientation, for example, with pressure pulses from the surface pump. It will be understood that orientation includes inclination, as much, and depth components. Typical rates of change of orientation of the drill string are 1-3 degrees per 100 feet of vertical depth. Hence, over a distance of about 3,000 feet, the drill string orientation can change from vertical to horizontal relative to the surface. A gradual change in the direction of the rotary drilled hole is necessary so that the drill string can move within the borehole and the flow of drilling fluid to and from the drill bit is not disrupted.
Another type of known drilling is coiled tubing drilling. In coiled tubing drilling, the drill string tubing is fed into the borehole by an injector assembly. In this method the coiled tubing drill string has specially designed drill collars located proximate the drill bit that apply weight to the drill bit via gravity pull. In contrast to rotary drilling, the drill string is not rotated. Instead, a downhole motor provides rotation to the drill bit. Because the coiled tubing is not rotated or used to force the drill bit into the formation, the strength and stiffness of the coiled tubing is typically much less than that of the drill pipe used in comparable rotary drilling. Thus, the thickness of the coiled tubing is generally less than the drill pipe thickness used in rotary drilling, and the coiled tubing generally cannot withstand the same rotational and tension forces in comparison to the drill pipe used in rotary drilling.
A known method and apparatus for drilling laterally from a vertical well bore is disclosed in U.S. Pat. No. 4,365,676 issued to Boyadjieff, et al. The Boyadjieff patent discloses a pneumatically powered drilling unit which is housed in a specially designed carrier, and the carrier and drilling unit are lowered to a desired position within an existing vertical well bore. The carrier and drilling units are then pivoted into a horizontal position within the vertical well bore. This pivotal movement is triggered by a person located at the surface who pulls a string or cable that is attached to one end of the carrier unit. From this horizontal position, the drilling unit leaves the carrier unit and begins drilling laterally to create an abrupt switch from a vertical to a lateral hole. The carrier is removed from the well bore once the drilling unit exists the carrier unit.
The drilling unit disclosed in the Boyadjieff patent discharges air near the drill bit to push the cuttings and rock chips created by the drilling process around the drilling unit. These cuttings are supposed to fall into a sump located at the bottom of the vertical well bore. This causes the bottom end of the vertical well bore to be filled with debris and prevents the use of the vertical well bore. The debris may also have a tendency to plug and fill the lateral hole. The drilling unit moves within the lateral hole by a series of teeth which are adapted to engage the sidewall of the lateral hole while the hole is being bored. These teeth transfer the drilling forces to the sidewalls of the hole to allow the drill bit to be pushed into the formation. The drilling unit is also connected to a cable guiding and withdrawal tool that is inserted into the vertical well bore to allow removal of the carrier and drilling unit from the lateral hole.
Another method and apparatus for forming lateral boreholes within an existing vertical shaft is disclosed in U.S. Pat. No. 5,425,429 issued to Thompson. The Thompson patent discloses a device that is lowered into a vertical shaft, braces itself against the sidewall of the vertical shaft, and applies a drilling force to penetrate the wall of the vertical shaft to form a laterally extending borehole. The device is generally cylindrical and includes a top section that is sealed to allow complete immersion in drilling mud. The top section also contains a turbine that is powered by the drilling mud. The bottom section of the device is open to the vertical shaft. The device is held in place within the vertical shaft by a series of anchor shoes that are forced by hydraulic pistons to engage the sidewall of the vertical shaft. These hydraulic pistons are powered by the turbine located in the top section of the device.
The device disclosed in the Thompson patent is anchored within the existing vertical shaft to provide support for the drilling unit as it drills laterally. The drilling unit uses an extendable insert ram to drill laterally into the surrounding formation. The insert ram consists of three concentric cylinders that are telescopically slidable relative to each other. The cylinders are hydraulically operated to extend and retract the insert ram within the lateral borehole. A supply of modular drill elements are cyclically inserted between the insert ram and the drill bit so that the insert ram can extend the drill bit into the surrounding formation. In operation, the drilling unit must be stopped and retracted each time the length of the insert ram is to be increased by inserting additional modular drill elements. The insert ram must then re-extend to the end of the lateral borehole to begin drilling again.
A further method for creating lateral bores is described in U.S. Pat. No. 5,010,965 issued to Schmelzer. The Schmelzer patent discloses a self-propelled ram boring machine for making earth bores. The system is operated using compressed air and is driven by a piston which triggers periodic blows by a striking tip.
U.S. Pat. No. 3,827,512 issued to Edmond discloses an apparatus for applying a force to a drill bit. The apparatus drives a striking bit, under hydraulic pressure, against a formation which causes the striking bit to form a borehole. In particular, the body of the apparatus is a cylinder containing two hydraulically operated pistons. Connected to the pistons are two anchoring assemblies which are located around the exterior surface of the tool. The anchoring assemblies contain a plurality of serrations and are periodically actuated to engage the sidewall of the borehole. These anchors provide support for the apparatus within the borehole such that a drill bit can be forced into the formation. The drill bit, however, can only be pushed in one direction. Additionally, the drill bit can only be periodically pushed into the formation because the apparatus must repeatedly unanchor and repressurize the piston chambers to move within the borehole.